Sub-carrier multiplexed broadcast optical networks are of interest because they offer a cost effective means of information transmission in local and larger area networks. In a sub-carrier multiplexing arrangement an optical carrier is intensity modulated by microwave sub-carriers. Data from each user or channel is used to modulate (e.g., ASK, PSK or FSK in the case of a digital system or by FM for an analog system) a separate microwave frequency sub-carrier. The term "sub-carrier multiplexing" is generally used to refer to microwave sub-carriers and optical carriers and to distinguish them from optical frequency division multiplexing. A high speed photodetector receives the sum of all transmitted subcarrier channels and the desired channel is selected with a microwave band pass filter or RF heterodyne receiver.
Investigations have shown that the components which make up a sub-carrier multiplex network suffer from non-linear effects which lead to distortion. The present invention is concerned with measurement techniques which enable the distortion in such a network to be evaluated. The overall distortion of such a network is made up of a number of contributions, typically from the up and down conversion mixers, laser drive, laser, the receiver and, in some instances, a laser amplifier.
Distortion measurement techniques are known in frequency division multiplex systems and operate by using standardized white noise generators. A white noise test signal is input at base band or at an intermediate frequency with a measurement slot created by a filter. Intermodulation products created by the channel are transmitted in the measurement slot and detected at a receiving terminal. This technique could be applied to SCM optical systems, but it provides little information regarding the distortion characteristics of individual system components. Additional difficulties are introduced when full band width loading is required as there is a need for an array of frequency up/down converters, each with white noise loading and an appropriate tracking filter.